I’m gonna make a bold prediction. You could
watch this video 5 seconds after it’s posted and someone will already have commented the
mitochondria is the powerhouse of the cell. It’s pretty clear how these little organelles
became a meme. It’s got to be the most repeated line in biology and has been firmly inserted
into our middle school textbooks for years. There’s just something catchy to it. Sidenote,
do powerhouses say they’re the mitochondria of wherever? If you’re in charge of the
PR for a powerhouse anywhere in the world, DM us. That whole powerhouse nickname came
from the mitochondria’s energy production capabilities, but it’s so much more than
that. Today we’ll go over how this little powerhouse powers our cells, where it came
from, and some new research that you probably didn’t hear about back in your middle school
textbook. You may remember some variation of this diagram, a classic animal cell with
a little jellybean-shaped organelle called the mitochondrion. Mitochondrion for singular,
mitochondria for plural. That diagram is fine when you’re learning the structures for
the first time, but of course, a real life cell is more complex. That little jellybean
shape is only one of the possible shapes a mitochondrion can take. What it looks like
can be different from cell to cell. Plus, you don’t just have a handful of mitochondria
per cell, you have hundreds to thousands of them floating in your cells. And even that
number depends on what type of tissue we’re talking about. Like a skeletal muscle cell
might be 3 to 8 percent mitochondria by volume but a liver cell could be about 20 percent.
Meanwhile, heart muscle cells are laughing at those numbers because they’re about 35
- 40 percent mitochondria by volume. They win by a long shot. Now, all mitochondria
do have some structural things in common. They each have two membranes — one outer layer,
one inner layer, and some space in between them.That outer membrane works like a protective
but permeable layer, letting different compounds in or out of the mitochondrion. Meanwhile
the inner membrane is where some important biology happens to manufacture ATP. This is
the molecule that fuels our major biological processes, so it’s often called energy currency.
We’re going to get into more depth on that whole ATP thing in the next video, but this
inner layer, as well as the matrix within the mitochondria, is where the cells generate
most of their ATP. Zooming back out to the mitochondrion as a whole, it looks almost
like a separate cell in its own right. That’s because at one point, it was. The most widely
accepted theory of how we got these little guys is the endosymbiosis theory. Endo- meaning
into, -symbiosis meaning living together — this word means that one cell engulfed another
cell and it resulted in a mutually beneficial relationship. About 3.8 billion years ago,
earth’s atmosphere didn’t have oxygen in it, and the only things living on our planet
were single celled organisms that were anaerobic, meaning they didn’t need oxygen to survive. Fast
forward about six hundred million years and photosynthetic bacteria were everywhere, taking
sunlight and a few other ingredients and cranking out oxygen as a byproduct. A few hundred million
years later, those photosynthetic bacteria had produced so much oxygen that it fundamentally
changed the composition of Earth’s atmosphere. Here’s the thing, oxygen was actually toxic to those anaerobic
cells. It’s so bizarre that something we need on a daily basis was so deadly back then. It’s
like if I found out my ancestors were allergic to tacos. That meant that these anaerobic
bacteria were at a huge disadvantage once the atmosphere was made of, what was to them,
poison gas. By two and a half billion years ago, a new type of bacteria started showing
up in the fossil record. These bacteria were aerobic, meaning they could use oxygen, and
it even helped them create energy. That is an excellent evolutionary advantage when the
atmosphere is made of a gas you can use. The theory suggests that eventually, one of those
anaerobic single celled organisms consumed an aerobic purple bacteria that survived being
eaten and they kicked off a symbiotic relationship. That was the first mitochondria. That purple
bacteria could consume and metabolize oxygen, which provided energy for the host cell. And
in return, the host cell protected the bacteria. We still don’t totally know the conditions
around that moment of symbiosis, but we have fossil evidence of it starting about one and
a half billion years ago. The result of this ancient endosymbiosis is today’s powerhouse
of the cell. We kept mitochondria around to make energy for our cells. And since they
were once their own separate organisms, they retained certain features of their past selves,
one of which was their genetic information. Just like our larger cells, mitochondria need
certain proteins to do their jobs, so they need genes to tell them what proteins to make. Our
DNA, which makes up our genes, is kept in our cell’s nucleus, what I’ll call nuclear
DNA for the rest of the episode. Some of our nuclear DNA makes proteins for the mitochondria,
then ships them out for it to use. But the mitochondria also has its own DNA, separate
from the DNA in your cell’s nucleus. Plus, it has the cellular machinery to make new
mitochondrial proteins, again, separate from the rest of your cell. This is the mitochondrial
genome, or the entirety of its genetic information, and it’s much smaller than the genome in
the cell’s nucleus. It’s a small circle with only about sixteen thousand base pairs
while the nuclear genome has billions of base pairs. Now, the vast majority of proteins
that get used by the mitochondria come from nuclear DNA, but that mitochondrial DNA lets
us make some cool observations. Thanks to sexual reproduction, humans are genetic mishmashes
of our parents, so you might expect that our mitochondrial DNA comes from our parents too. As
a matter of fact, for a few reasons, we only inherit mitochondrial DNA from our mothers.
When you were first developing in utero, most of the chromosomes from your biological parents
recombined to form your chromosomes. This is part of what makes you physically different
than your parents. But mitochondrial DNA, as well as the Y chromosome from your father,
don’t recombine so they get used to study lineage. This kind of DNA does mutate, but
it’s otherwise well conserved, so the information in our mitochondria’s genes are similar
to our maternal ancestors way way back in the past. Sequencing that mitochondrial DNA
and comparing genomes has allowed researchers to trace people back to a single female ancestor
in Africa thousands of years ago, and follow human migration. Now, why does mitochondrial
DNA only come from your mother? Good question! The first is that egg cells hold way more
mitochondrial DNA than sperm cells, it’s around two hundred thousand molecules in an
egg cell and like, single digits, in sperm cells. Some estimates are a little higher,
but the point remains — egg cells outnumber sperm by a lot when it comes to mitochondrial
DNA. Plus, sperm store most of their mitochondria in their metabolically active tails. It does
take a lot of energy to swim, after all. Now, aside from helping your cells make energy
and providing clues about our ancestry, ongoing research is showing us some new features of
our mitochondria. For example, research by scientists at the Salk Institute showed that
mitochondria can kick off a series of events that signal the rest of the cell that it’s
under stress — the kind of chemical stress that can damage DNA. This phenomenon caught
their attention when they observed how defective mitochondrial DNA caused the cell to eject
the damaged mitochondria and actually send out a chemical warning signal that strengthens
the cell’s defenses. So they investigated what would happen if any of that DNA spilled
out of the mitochondria and into the liquid around it. When they did, they saw that a certain
set of genes were activated that usually activate when there’s an invading virus. Awesome,
that’s exactly what we want our immune system to do, attack a virus when it detects one. Now,
that same set of genes is also activated by chemotherapy-resistant cancer cells. Specifically,
cancer that’s resistant to doxorubicin, a chemotherapy drug that attacks nuclear DNA.
When they studied this drug more closely, they found that it caused the release of mitochondrial
DNA from the mitochondria, which activated a subset of those protective genes, which
then protected the nuclear DNA. The point of this drug was to attack nuclear DNA, but
when these genes were activated, it set up a pathway to defend the nuclear DNA, which
explains why some cancers were resistant to the drug. The researchers took cancer cells
and induced stress on their mitochondrial DNA, and as expected, they activated more
of those genes and developed a resistance to doxorubicin afterwards. This research
doesn’t show that doxorubicin is a useless chemotherapy drug, it just explains why some
cancers develop resistance to the drug. They think the purpose of that response is to protect
the DNA in the cell’s nucleus, making the mitochondria a warning signal that something
bad is happening. They hope that if they can find a way to protect the mitochondrial
DNA, they’ll prevent that immune response within the cell and find more effective chemo
treatments. So not only is our powerhouse of the cell effective in generating energy,
it’s got a fascinating backstory, with clues to our past and to our future medical treatments.
If you’re wondering why we kind of glossed over the energy generating aspects of the
mitochondria, it’s because we’re saving it for later. Check out the next episode in
the series to learn about how our cells generate energy. Yep, I’m gonna try to teach ya’ll
how ATP works without making you fall asleep from boredom. Wish me luck. I’m Patrick Kelly, thanks for watching this episode of Seeker Human.
